1. Field of the Invention
The present invention relates to a sheet conveying apparatus equipped in an image forming apparatus such as a copying machine, a printer, and a facsimile.
2. Description of the Related Art
In the prior art, an image forming apparatus such as a copying machine, a scanner, and a printer uses a sheet conveying apparatus (registration apparatus) provided immediately in front of an image forming portion of the image forming apparatus and used for controlling the posture and position of a sheet. For example, there have been proposed a loop registration method and a shutter registration method as a cost effective method. In the loop registration method, a leading edge of a sheet is abutted against a nip portion of a stopped roller pair so that the sheet is deflected, whereby skew feeding is corrected by the elasticity of the sheet.
Recently, in the image forming apparatus, the productivity of image formation is required to be improved. The productivity means the number of times of image formation per a unit time. High productivity can be obtained by reducing an interval between sheets (hereinafter referred to as a “sheet interval”). However, in the loop registration method and the shutter registration method, since the time of abutting the sheet leading edge against a blocking object to stop the sheet temporarily is required, the sheet interval corresponding to the stop time is increased. Thus, in order to reduce the registration time, an active registration method has been proposed.
In the active registration method, the skew feeding of a sheet is corrected by controlling a sheet conveying speed of skew feeding correction rollers which are arranged on the same axis as a sheet skew feeding amount detecting portion, the axis being perpendicular to a sheet conveying direction, and each independently driven. According to the active registration method, the skew feeding can be corrected while a sheet is conveyed, and the sheet interval can be reduced in comparison with other registration methods.
In the active registration method, in order to increase the skew feeding correction accuracy, there has proposed such a constitution that a conveying roller having a notch is rotated once to be controlled, so that the skew feeding is corrected simultaneously with phase control (see, U.S. Patent Application Publication No. 2008/0006992 A1).
However, a corner of a sheet is not always cut at a right angle, and in some types of sheets, the corners are not right angle. The degree of right angle of the corner of a sheet is determined when the sheet is cut, and therefore, when sheets are cut for each lot, the corners of the sheets have substantially the same angle in the same cutting lot. Accordingly, even if the skew feeding correction accuracy is increased by controlling the phase of the skew feeding correction roller, under the influence of the right angle degree of the sheet corner, positional deviation between front and back surface images (hereinafter referred to as “front and back deviation”) may occur for example when images are formed on the both sides of a sheet.
Currently, in printing industry, deliverables in which the positions of the front and back surface images coincide with each other with high accuracy at low cost are required. With regard to the front and back deviation caused by the right angle degree of the sheet corner, a user regulates the skew feeding amount for each of the front and back surfaces to correspond to the positional alignment between the front and back surface images. However, a user should perform the operation of aligning the positions of the front and back surface images with high accuracy, so that trouble and a lot of time to perform the operation are required.
In the positional alignment between the front and back surface images by a user, the user outputs a sample image and then confirms the positional relationship between the sample image and an end of a sheet, and the user per se should calculate a sheet skew feeding regulation amount that does not cause the front and back deviation. The user inputs the calculated skew feeding regulation amount through an operation portion to output the sample image again, and, thus, to confirm that the sheet and the sample image have a desired positional relationship. In order to perform such an operation with high accuracy, the same operation needs to be repeated, so that trouble and a lot of time are required. Further, as described above, the cutting lot of a sheet and the right angle degree of the sheet corner are correlated, and therefore, when the positions of the front and back surface images are aligned with high accuracy, a user is required to perform the above operation for each cutting lot.
Furthermore, a user is required to perform two types of skew feeding regulations according to the use of the sheet for the following reason. Usually, the skew feeding is regulated so that an image is parallel to the long side of a sheet (the side in the sheet conveying direction) amongst the short and long sides forming the corner of the sheet. However, when a sheet processing apparatus such as a finisher, which applies processing to a sheet, is attached downstream from a discharge opening of an image forming apparatus, other methods may be employed. In the sheet processing apparatus, staple processing and punching processing are performed with high accuracy by mechanically abutting the leading edge in the sheet conveying direction of a sheet. Accordingly, when the sheet conveying direction leading edge of the sheet is the short side forming the corner of the sheet, it is preferable that the skew feeding is regulated so that an image is parallel to the short side of the sheet (the leading edge side). Accordingly, with regard to a continuous form of sheet whose long side corresponds to the sheet conveying direction, the skew feeding regulation amount should be set so that the short side of the sheet is parallel to an image. Thus, according to presence of the sheet processing, a user should separately perform two types of skew feeding regulations, i.e. based on the short side of the sheet and on the long side basis. Consequently, it takes time and trouble.
In view of the above problems, the present invention provides a sheet conveying apparatus that reduces the trouble (burden) required for sheet skew feeding regulation by a user even when a corner of a sheet is not right angle.